This application relates generally to communication systems, and, more particularly, to wireless communication systems.
Wireless communication systems typically deploy numerous access points (or other types of devices for providing wireless connectivity such as base station transceivers) for providing wireless connectivity to access terminals (or other types of user equipment, mobile units, or wireless-enabled devices). Each access point is responsible for providing wireless connectivity to the mobile units located in a particular cell or sector served by the access point. In some cases, the mobile units may initiate wireless communication with one or more access points in the network, e.g., when the user of the mobile unit would like to initiate a voice or data call. Alternatively, the network may initiate the wireless communication link with the mobile unit. For example, in conventional hierarchical wireless communications, a server transmits voice and/or data destined for a target mobile unit to a central set of network elements in a wireless network. The core network elements may then transmit paging messages to the target mobile unit via one or more access points. The target mobile unit may establish a wireless link to one or more of the access points in response to receiving the page from the wireless communication system. A radio resource management function within the Radio Access Network (RAN) and wireless core network receives the voice and/or data and coordinates the radio resources and time resources used by the set of access points to transmit the information to the target mobile unit.
One alternative to the conventional hierarchical network architecture is a distributed architecture including a network of access points, such as a base transceiver station (BTS) that implements distributed communication network functionality. For example, each base transceiver station may combine core network functions in a single entity that manages radio links between one or more mobile units and an outside network, such as the Internet. The base transceiver stations wholly encapsulate the cellular access technology and may proxy functionality that utilizes core network element support to equivalent IP functions. For example, IP anchoring in a UMTS base transceiver station may be offered through a Mobile IP Home Agent (HA) and the GGSN anchoring functions that the base transceiver station proxies by through equivalent Mobile IP signaling. Compared to hierarchical networks, distributed architectures have the potential to reduce the cost and/or complexity of deploying the network, as well as the cost and/or complexity of adding additional wireless access points to expand or enhance the coverage of an existing network. Distributed networks may also reduce (relative to hierarchical networks) the delays experienced by users because packet queuing delays at the wireless core of hierarchical networks, e.g. delays at the RNC in a UMTS network or at the packet data serving node (PDSN) of a CDMA network, may be reduced or removed.
At least in part because of the reduced cost and complexity of deploying a base transceiver station, these devices may be deployed in locations that are impractical for conventional base stations. For example, small cells that provide a smaller coverage area can be deployed in conjunction with (and often event overlay configuration with) a macrocell that provides a relatively larger coverage area. Small cells may be deployed in a residence or building to provide wireless connectivity to the occupants of the residents of the building. Small cells deployed in a residence are typically referred to as home base station routers or femtocells because they are intended to provide wireless connectivity to a smaller coverage area or cell that encompasses a residence. Base transceiver stations deployed for public or semi-private use may also be referred to as metro cells. However, the functionality in the different types of small cells is typically quite similar to the functionality implemented in a conventional macrocellular base transceiver station that is intended to provide wireless connectivity to a macro-cell that may cover an area of approximately a few square kilometers. One important difference between a femtocell and a conventional base transceiver station is that femtocells are designed to be plug-and-play devices that can be purchased off-the-shelf and easily installed by a lay person. Deployment of femtocells and/or metro cells may result in a very large number of cells that overlap, overlay, and/or are encompassed by one or more macro-cells.
Active mobile units may be handed off from one base station to another as the mobile units wander throughout the wireless communication system. Mobile units may also be handed off from a macro cellular base station to a home base station router or femtocell, even when the coverage area of the base station completely encompasses the coverage area of the femtocell. For example, a user's mobile unit may hand off to a home base station router when the user enters the radio environment of the femtocell and the signal strength achieves a specific quality threshold. From the point of view of the user, robust handover techniques are critical for supporting seamless service as the mobile unit wanders. Users quickly become frustrated by gaps or silences in voice communication that may be caused by latency in the handover process. Some users may even switch providers if calls are frequently dropped when the user wanders from one cell to another. Idle mobile units may autonomously select a new cell and perform a cell reselection process to camp on the new cell.
The basic condition for initiating a handover or cell reselection is that the signal strength from the candidate target base station or cell is stronger/better than the signal strength from the current serving base station or cell. However, simply handing off a mobile unit as soon as the target base station appears to have a stronger signal than the serving base station can lead to a number of problems. For example, the signal strengths near the boundaries between a serving cell and its neighbor cells are (almost by definition) nearly equal. Furthermore, the relative values of the signal strengths may be subject to rapid fluctuations when both signals are close to the threshold. The signal strength received by each mobile unit near a boundary is therefore approximately equal and relatively small deviations can cause the relative signal strengths to flip-flop. The strength of the signals received by a particular mobile unit may also vary rapidly due to movement of the mobile unit and/or environmental changes. Consequently, the mobile unit may be rapidly handed back and forth (a phenomenon known as ping-ponging or hysteria) if the hand off is performed based only on the relative signal strength. Ping-ponging consumes valuable overhead unnecessarily, degrades the perceived call quality, and can even lead to dropped calls.
Handovers and/or cell reselection can be made more robust by using a more sophisticated handoff condition. For example, conventional handovers are performed when the signal strength from the candidate cell is better than the signal strengths from the current serving cell by a certain amount determined by a hysteresis value and offset (or bias) values. Each cell uses a single value of the hysteresis, e.g., 2 dB. Each cell also maintains different values for the offset that are applied to handoffs between the cell and its neighbor cells. For example, the offset value for handoffs between a serving cell and a first neighbor cell may be 1 dB and the offset value for handoffs between the serving cell and a second neighbor cell may be 2 dB. A time-to-trigger (TTT) is used to delay the hand off until the “better” conditions on the target cell persist for at least the TTT duration. In 3G technologies, the hysteresis, offset values, and TTT are set to one golden set that is applied to all cells. The golden set is selected for convenience alone and does not provide performance benefits.